Svelte 3, Babel 7, Polyfill IE 11

babel.config.js

// https://www.learningsomethingnew.com/how-to-make-your-svelte-3-cordova-app-work-on-old-phones-using-babel-7-and-rollup
// https://blog.az.sg/posts/svelte-and-ie11/
module.exports = function (api) {
  api.cache(true);

  const presets = [
    [
      '@babel/preset-env', {
        useBuiltIns: 'usage',
        corejs: 3,
        targets: {
          browsers: 'last 2 versions'
        }
      }
    ]
  ];

  return {
    presets
  }
}

package.json

{
  "name": "babel-ie11",
  "version": "0.0.1",
  "scripts": {
    "build": "npm install && rollup -c",
    "dev": "npm install && rollup -c -w",
    "start": "sirv public --single"
  },
  "devDependencies": {
    "@babel/core": "^7.10.2",
    "@babel/preset-env": "^7.10.2",
    "@rollup/plugin-babel": "^5.0.3",
    "@rollup/plugin-commonjs": "^13.0.0",
    "@rollup/plugin-node-resolve": "^8.0.1",
    "core-js": "^3.6.5",
    "rollup": "^2.16.1",
    "rollup-plugin-livereload": "^1.3.0",
    "rollup-plugin-svelte": "^5.2.2",
    "rollup-plugin-terser": "^6.1.0",
    "svelte": "^3.23.2"
  },
  "dependencies": {
    "sirv-cli": "^1.0.0"
  }
}

rollup.config.js

import svelte from 'rollup-plugin-svelte';
import resolve from '@rollup/plugin-node-resolve';
import commonjs from '@rollup/plugin-commonjs';
import livereload from 'rollup-plugin-livereload';
import { terser } from 'rollup-plugin-terser';
import babel from '@rollup/plugin-babel';

const production = !process.env.ROLLUP_WATCH;

export default {
  input: 'src/main.js',
  output: {
    sourcemap: true,
    format: 'iife',
    name: 'app',
    file: 'public/build/bundle.js'
  },
  plugins: [
    svelte({
      dev: !production,
      css: css => {
        css.write('public/build/bundle.css');
      }
    }),
    resolve({
      browser: true,
      dedupe: ['svelte']
    }),
    commonjs(),

    !production && serve(),
    !production && livereload('public'),

    production && babel({
      babelHelpers: 'bundled',
      extensions: [
        '.js',
        '.mjs',
        '.html',
        '.svelte'
      ],
      exclude: [
        /\/core-js\//
      ]
    }),
    production && terser()
  ],
  watch: {
    clearScreen: false
  }
};

function serve() {
  let started = false;

  return {
    writeBundle() {
      if (!started) {
        started = true;

        require('child_process').spawn('npm', ['run', 'start', '--', '--dev'], {
          stdio: ['ignore', 'inherit', 'inherit'],
          shell: true
        });
      }
    }
  };
}